Amino acids content and digestibility in the feedstuffs

The first and only direct way to assess amino acids contents in feedstuffs is to analyze samples using specific methods. Amino acids contents can be measured with a good precision using reference methods in a renowned laboratory. The repeatability and reproducibility levels are at least similar to those of proximal analyses. In case analyses cannot be performed, the formulator may need indirect evaluations of amino acids contents. In our technical bulletin N°32, table values of the amino acids contents of 27 ingredients and linear regressions between nitrogen and individual amino acids have been established on the basis of the Ajinomoto Animal Nutrition Europe laboratory database.

Contrary to the feed-use amino acids which are completely digestible, the amino acids supplied through the dietary proteins can not be entirely absorbed by the animal's digestive tract. The amount of each amino acid that can be absorbed depends on the species, on the type of raw material used (the variety, the structure of the proteins, the content in dietary fibre, etc.) and on the amino acid under consideration. The most effective way to formulate feeds for monogastrics is thus by using the digestible amino acids values rather than the total ones (Figure 1). This results in a more accurate supply to meet animal requirements. Knowledge of the digestibility of each amino acid in each raw material is therefore essential for the adequacy of the dietary supply of amino acids.

The notion of digestibility has been gradually developed on both, methodological and conceptual levels. Digestibility, which was initially measured at the fecal level, is now measured at the ileal level to account for the changes caused by the microflora of the large intestine. On the one hand, undigested amino acids on the ileal level can be catabolised by the microflora of the large intestine or used for the synthesis of microbial protein. On the other hand, some amino acids are synthesized de novo by bacteria without being absorbed by the animal; these eventually end up in the feces. The proportion of nitrogen of microbial origin at the fecal level may represent 65% to 90% of total fecal nitrogen. Ileal digestibility therefore represents the absorption of amino acids in the digestive tract far better than fecal digestibility.

However, the profile of amino acids at the end of the small intestine is not only the result of the absorption of dietary amino acids. The digestive process is also accompanied by secretions of digestive juices, mucins, immunoglobulins and cellular desquamations, whose protein elements blend with the proteins of dietary origin in the intestinal lumen. These secretions are partially digested and reabsorbed but some of them, representing between 10 and 80% of the nitrogen present in the ileal digesta, escape this re-absorption. This fraction of endogenous origin can be measured by feeding the animal with a protein-free diet. Although this technique only enables basal endogenous or non-specific losses to be measured, it makes it possible to propose corrected apparent ileal digestibility (AID) coefficients which are then called true or standardized ileal digestibility (SID) coefficients, meaning corrected from the basal endogenous losses (Figure 1).

The main tables which have been published and are currently available for the formulation give information about SID coefficients to formulate the content of amino acids in feeds as closely as possible to their availability for animals. The EvaPig® software, which uses information from the INRA-AFZ tables of feedstuffs composition, is the only one that provides an interactive interface to estimate the amino acids content of a wide range of feedstuffs, their digestibility and their net energy content.